A. Field of the Invention
The present invention relates to electrochemical cells and more particularly to a corrosion resistant battery with a zinc powder anode and an alkaline or an acidic electrolyte, and to a method for its manufacture. Applicants' novel battery is corrosion-resistant and combines the cost benefits and efficiency of a zinc anode while avoiding the hazards normally associated with adding mercury to the zinc in order to suppress corrosion. The battery is manufactured by compacting zinc powder to a density of about at least 6.5 g/cc.
B. Description of the Prior Art
Zinc was the first and also the most common material used as a negative electrode when converting electrochemical energy in both non-rechargable and rechargable batteries. Zinc metal offers a very attractive range of properties for battery application, including low cost, no toxicity, ease of fabrication, high energy density, low electronegativity (high cell voltage) and high exchange current density.
When zinc metal is in contact with an aqueous alkali, anodic dissolution of zinc metal and cathodic evolution of hydrogen gas occur simultaneously. The former reaction causes zinc oxide or zinc hydroxide to form. The oxides then react with excess hydroxide in the bulk electrolyte to form a soluble complex anion called zincate (Zn(OH).sub.4).sup.-2 : EQU Zn+4OH.sup.- .fwdarw.(Zn(OH).sub.4).sup.-2 +.sup.2e- ( 1)
At the same time, water is reduced to hydrogen gas: EQU 2H.sub.2 O+2e-.fwdarw.H.sub.2 +2OH.sup.- ( 2)
This combination of reactions institutes the corrosion reaction with the evolution of hydrogen gas: EQU Zn+2OH.sup.- +2H.sub.2 O.fwdarw.Zn(OH).sub.4.sup.-2 +H.sub.2 ( 3)
The self discharge reactions described above are detrimental, not only because they reduce the dischargeable capacity (energy) of the battery with time, but also because they require incorporation of a hydrogen gas venting system. This, in turn, makes the battery more prone to deterioration by means of evaporation of the electrolyte.
It has been a common practice for many years in zinc battery technology to add metallic mercury to the zinc electrode to suppress the evolution of hydrogen gas. Mercury-zinc alloy (zinc amalgam) has a lower corrosion rate than pure zinc. Mercury, however, is both highly toxic and volatile. Unusual safety precautions are necessary during the manufacture of products containing mercury. An additional problem associated with the use of mercury is the disposal of products containing that toxic substance. Consequently, complex disposal techniques must be employed for products containing more than 0.2 ppm of mercury to reduce the risk of ground water contamination.
The elimination of mercury from commercial batteries utilizing zinc as the anode is, therefore, considered to be a desirable objective. The advantages that result from the elimination of mercury include cost savings associated with the processing of zinc anodes and the disposal of the spent battery materials.
There have been other attempts in the art to mitigate the hazards and added expense associated with the evolution of hydrogen gas in electrochemical cells with zinc electrodes. Those attempts have focused upon the use of additives, either to the alkaline electrolyte solution or to the zinc anode itself. For instance, U.S. Pat. No. 4,377,625 to Parsen et al discloses the addition of aminocarboxylic acid, polyamine or aminoalcohol chelating agents which are said to combine by a coordinate bond with the zinc in the electrolyte solution. However, it is unclear how such bonding would inhibit corrosion of the anode and the consequent evolution of hydrogen gas.
U.S. Pat. No. 3,580,740 to James discloses a pressed powder zinc electrode with the addition of from about 1 to 10% by weight lead sulfide to reduce hydrogen gassing at the zinc electrode.
James' experiments reveal, however, that the addition of lead sulfide to the zinc anode was not as effective in reducing hydrogen gas evolution as is the addition of mercuric oxide. Indeed, the use of a mercury additive in the anode is the most common approach in the art for the purpose of inhibiting corrosion and hydrogen gas evolution. See McBreen, Electrochimi., Icta, 26: 1439-1446 (1981); U.S. Pat. No. 3,870,564; U.S. Pat. No. 4,339,512. Several research projects have concluded that mercury is the only effective additive in increasing electrode discharge capacity under all discharge conditions, e.g., Dirkse and Shoemaker, J. Electrochemical Society, p. 115 (August, 1968); Shepard, "Silver Zn Alkaline Prim. Cell", part IV, NRC Report 4885, Naval Research Lab, Washington, D.C. (February, 1957). As stated above, a mercury additive to anodes is extremely toxic and volatile. Thus, unusual safety precautions must be taken at both the manufacture and disposal stages.